The present invention relates to a dynamic damper and a dynamic damper-installed tennis racket for improving shock and vibration characteristic thereof which are generated when we hit a ball with the tennis racket.
The dynamic damper (vibration-damping member) having a viscoelastic part and a mass-adding part connected to the viscoelastic part is often used to reduce and relieve shock and vibrations generated in sports ball-hitting goods and tools when they are used. In the tennis rackets disclosed in Japanese Patent Publication No.52-13455 and Japanese Patent Application Laid-Open Nos.52-156031 and 4-263876, the cantilevered dynamic damper having a load-applying material fixed thereto through the elastic material is installed on the tennis racket. The dynamic damper resonates with vibrations of the racket frame to damp its vibration.
More specifically, in Japanese Patent Publication No.52-13455, as shown in FIG. 23, the cantilevered damper 6 consisting of the long and narrow elastic material is installed at the end of the grip 5. The base of the steel wire 6b having a weight 6a installed on its front end is embedded in the racket frame. In Japanese Patent Application Laid-Open No.52-156031, as shown in FIG. 24, the base 3a of the dynamic damper is fixed to the throat 4 of the tennis racket, and the body 3c of the dynamic damper is connected to the base 3a via the neck 3b to vibrate the body 3. In Japanese Patent Application Laid-Open No.4-263876, as shown in FIG. 25, the load-applying member 4b is fixed to the grip end 5a of the tennis racket via the elastic member 4a. 
In the above-described proposed conventional tennis rackets, to mainly suppress the 1st vibration of the racket frame in the out-of-plane direction (direction perpendicular to the gut plane of the racket frame), the configuration of the dynamic damper and the fixing position thereof are designed. That is, the load-applying material is fixed to the front end of one elastic material whose one end is fixed to the racket frame. The dynamic damper vibrates at the same frequency as that of the racket frame, thus consuming energy and reducing and damping the vibration and shock of the racket frame rapidly.
However, a player feels not only the vibration of the tennis racket in the out-of-plane direction but also the vibration thereof in the in-plane direction (widthwise direction of the racket frame parallel with the face of the racket frame). When the player hits a ball with at a position apart from the axis of the racket frame, the player feels a shock generated by the rotation of the grip very unpleasant.
The vibration in the in-plane direction has not been considered much. The vibration of the gut-stretched part in the in-plane direction is generated by deformation of the gut which hits the ball directly, thus giving a big influence on the player""s evaluation on her/his ball-hitting feeling, namely, on whether the player feels good or bad when the player hits the ball with the tennis racket.
It is said that a so-called large racket having a large face area (area of gut-stretched part) developed to fly the ball a long distance generates more unpleasant vibration than a tennis racket having a small face area. This is because the large racket is liable to flex in the in-plane direction owing to the large face area. That is, the vibration of the gut-stretched part in the in-plane direction is large. From these facts, in the tennis racket such as the large racket designed to fly the ball a long distance, it is important to reduce the vibration in the in-plane direction in addition to the vibration in the out-of-plane direction.
Therefore the present applicant proposed a sectionally U-shaped dynamic damper as disclosed in Japanese Patent Application Laid-Open No.10-340836. The dynamic damper having the configuration can favorably reduce the vibration of the gut-stretched part in the in-plane direction in addition to its vibration in the out-of-plane direction.
The sectionally U-shaped dynamic damper can increase the vibration-damping performance in the in-plane direction in addition to that in the out-of-plane direction. But there is a case in which the effect for damping the vibration of the racket frame in the out-of-plane direction is smaller than that for damping the vibration thereof in the in-plane direction. To improve the vibration-damping performance, the dynamic damper has a room for improvement.
The present invention has been made in view of the above-described problems. Thus it is a first object of the present invention to provide a dynamic damper superior in relieving and reducing shock and vibrations.
It is a second object of the present invention to provide a dynamic damper-installed tennis racket reducing vibrations of the racket frame in the in-plane direction in addition to the out-of-plane direction and having a vibration damping factor of not less than 1% in the in-plane direction and in the out-of-plane direction to reduce burden on a player""s arm and allow a player to have a favorable feeling when the player hits a ball with the tennis racket.
To achieve the object, according to the present invention, there is provided a dynamic damper which comprises a viscoelastic part and a mass-adding part integrally layered on the viscoelastic part and is installed on a racket. The dynamic damper has a horizontal frame and a vertical frame disposed at both sides of the horizontal frame in the shape of a lattice. In the construction, the horizontal frame and the vertical frame are integrally formed or formed by joining the horizontal frame and the vertical frame separate from each other, the horizontal frame is installed on at least one surface of the racket in a thickness direction thereof, and the vertical frame is installed on both surfaces of the racket in a widthwise direction thereof.
The thickness direction of the racket means the direction perpendicular to the gut-stretched surface thereof. The widthwise direction of the racket means the direction parallel to the gut-stretched surface.
The horizontal frame is bent in the shape of a letter xe2x80x9cUxe2x80x9d. One end of a bent portion disposed at both sides of the horizontal frame is integral with the vertical frame or joined therewith. The bent portion disposed at both sides of the horizontal frame is installed on both surfaces of the racket in its widthwise direction.
It is preferable that the number of the horizontal frames is not less than two and that the horizontal frames are disposed, with the horizontal frames sandwiching a gut insertion hole therebetween. Thus in the case where the dynamic damper has two horizontal frames, it is rectangular. In the case where the dynamic damper has three horizontal frames, it has the shape of a Japanese character xe2x80x9cxe2x80x9d. In the case where the dynamic damper has four horizontal frames, it has the shape of a Japanese character xe2x80x9cxe2x80x9d.
That is, the lattice-shaped dynamic damper of the present invention has the long and narrow vertical frame integral with the horizontal frame or separate vertical frame and horizontal frame are jointed with each other.
As described above, the horizontal frame and the vertical frame are continuous and integral with each other and disposed in the shape of a lattice. Therefore in the dynamic damper-installed racket, the vertical frame resonates mainly with vibrations of the racket frame in a out-of-plane direction, whereas the horizontal frame resonates mainly with vibrations of the racket frame in a in-plane direction, thus effectively reducing vibrations in the out-of-plane direction and the in-plane direction. That is, because the horizontal frame and the vertical frame are disposed in the shape of a lattice, the dynamic damper has improved vibration-damping performance, thus reducing of shock and vibrations.
In the case where the dynamic damper is formed monolithically in the shape of a lattice, i.e., in the case where the vertical frame and the horizontal frame are formed integrally with each other in the shape of a lattice, the entire lattice resonates with the vibration of the racket frame in the in-plane direction, thus having an effect of reducing the vibration in the in-plane direction. That is, in the case where the horizontal frame and the vertical frame are formed integrally with each other in the shape of a lattice, the weight of the entire dynamic damper contributes to the reduction of the vibration of the racket frame in the in-plane direction and the out-of-plane direction, thus having a higher vibration reduction effect than that brought about by the horizontal frame contributing to the reduction of the vibration of the racket frame in the in-plane direction and the vertical frame contributing to the suppression of the vibration thereof in the out-of-plane direction. That is, the dynamic damper having the construction is superior in its vibration-damping performance.
Favorably the ratio of the length (L2) of the vertical frame to the length (L1) of the horizontal frame is not less than 0.3 nor more than 1.0. This is because if the ratio L2/L1 is less than 0.3, the region for vibrating the dynamic damper in the out-of-plane direction is so small that the effect of damping the vibration of the racket frame in the out-of-plane direction is small. On the other hand, if the ratio L2/L1 is more than 1.0, the weight of the entire dynamic damper becomes large. Thus it is difficult for a player to swing the tennis racket and for the entire dynamic damper to vibrate in the in-plane direction in particular. More favorably the ratio L2/L1 is not less than 0.6 nor more than 0.9. The length of the horizontal frame and the vertical frame means the length thereof at the center of the thickness of the dynamic damper.
It is preferable that the longitudinal direction of the vertical frame is set along the longitudinal direction (the longitudinal direction of the horizontal frame is perpendicular to the gut plain of the racket frame) of the racket frame. In this case, the dynamic damper can display excellent vibration-damping performance.
It is favorable that the width of each portion of the viscoelastic part is not less than 4 mm nor more than 8 mm. If the width of the viscoelastic part is less than 4 mm, the viscoelastic part is narrow in order to adhesion to an object such as the racket frame on which the viscoelastic part installed. If the width of the viscoelastic part is more than 8 mm, the dynamic damper is heavy. Consequently the vibration of the dynamic damper is bad, which causes the dynamic damper to have a low effect of damping the vibration of the racket frame. It is more favorable that the width of the viscoelastic part is not less than 4 mm nor more than 6 mm.
It is favorable that the thickness of each portion of the viscoelastic part is set to not less than 2.5 mm nor more than 5.5 mm. If the thickness of the viscoelastic part is set to less than 2.5 mm, it is difficult for the viscoelastic part to vibrate. On the other hand, if the thickness of the viscoelastic part is set to more than 5.5 mm, the viscoelastic part may be an interference when the dynamic damper is installed on the racket frame or the dynamic damper looks unattractive. It is more favorable that the thickness of each portion of the viscoelastic part is set to not less than 3 mm nor more than 5 mm.
The total weight of the dynamic damper is set to not less than 8 g nor more than 23 g. If the total weight of the dynamic damper is set to less than 8 g, the dynamic damper has an insufficient vibration reduction performance. On the other hand, if the total weight of the dynamic damper is set to more than 23 g, the racket frame has poor handling performance.
The complex modulus of elasticity of the viscoelastic part at 20xc2x0 C. and 10 Hz is set to not less than 0.3 MPa nor more than 1.5 MPa. A solid material having a complex modulus of elasticity less than 0.3 MPa and suitable for being installed on an object such as the tennis racket to which it is installed does not exist. On the other hand, if the complex modulus of elasticity of the viscoelastic part is more than 1.5 MPa, the frequency of the dynamic damper of the present invention is incapable of resonating with that of an object such as the tennis racket to which the dynamic damper is installed.
The complex modulus of elasticity of the mass-adding part at 20xc2x0 C. and 10 Hz is set to not less than 100 MPa nor more than 800 MPa.
A mass-adding part having a complex modulus of elasticity not less than 100 MPa nor more than 800 MPa is not as soft as the viscoelastic part but softer than a hard metal and elastic. Because the dynamic damper of the present invention is composed of the mass-adding part having a certain degree of softness and the very soft viscoelastic part, there is no fear that it hurts or injures a player""s hand even though it collides with the hand. That is, there is no fear that the dynamic damper deteriorates safety. In the case where the mass-adding part is soft, even though the viscoelastic part and the mass-adding part are integral with each other by the connection between the surfaces thereof, the viscoelastic part is not constrained strongly by the mass-adding part. Further the mass-adding part as well as the viscoelastic part is capable of deforming. Thus the entire dynamic damper generates a dynamic motion and a resonant phenomenon in a sufficient degree, thus sufficiently relieving and reducing shocks and vibrations.
If the mass-adding part is a hard rigid body, only the viscoelastic part is capable of deforming. When the viscoelastic part is integral with the mass-adding part by the connection between the surfaces thereof, the viscoelastic part is constrained strongly by the mass-adding part. Thus the dynamic damper is incapable of generating the dynamic motion and the resonant phenomenon sufficiently. Thus the dynamic damper is incapable sufficiently displaying an action of relieving and reducing shocks and vibrations.
If the complex modulus of elasticity of the mass-adding part of the dynamic damper of the present invention is less than 100 MPa, it is impossible to secure a specific gravity as the mass-adding part to have a sufficient mass-adding effect. On the other hand, if the complex modulus of elasticity of the mass-adding part is more than 800 MPa, the mass-adding part does not have a required degree of softness. If the complex modulus of elasticity of the mass-adding part is less than 300 MPa, the mass-adding part has a sufficient degree of softness. Thereby the dynamic damper is capable sufficiently displaying an action of relieving and reducing shock and vibrations. Therefore it is more favorable that the complex elastic modulus of the mass-adding part is less than 300 MPa.
The complex modulus of elasticity of the viscoelastic part and that of the mass-adding part are measured in the following conditions:
Measuring instrument: Viscoelastic Spectrum Graphy DVE-V4FT Rheospectrer manufactured by RHEOLOGY Corp.
Initial load: 250 g
Frequency: 10 Hz
Displacement amplitude: 5 xcexcm
Direction: pulling
Temperature: 20xc2x0 C.
Distance between chucks: 30 mm
It is preferable that the thickness of the entire dynamic damper, namely, the total thickness of the viscoelastic part and the mass-adding part is set to not less than 3.0 mm nor more than 7.0 mm. If the thickness of the entire dynamic damper is less than 3.0 mm, it is difficult for the dynamic damper to vibrate. If the thickness of the entire dynamic damper is more than 7.0 mm, the dynamic damper may be an interference when the dynamic damper is installed on the racket frame or the dynamic damper looks unattractive.
The mass-adding part of the dynamic damper of the present invention may be composed of metal. But it is preferable that the main components of the mass-adding part consist of metal powder having a high specific gravity and a highmolecular compound such as resin, rubber or elastomer and the mixture that the metal powder having a high specific gravity is dispersed in the macromolecular material.
In the case where the mass-adding part contains the highmolecular compound, the viscoelastic part of the present invention contains a highmolecular compound identical or similar to the highmolecular compound for the mass-adding part.
Elastic modulus-adjusting oil and/or moldability-improving oil may be added to the mixture of the metal powder having a high specific gravity and the highmolecular compound. A coloring pigment may be also added to the mixture.
The metal powder having a high specific gravity is not limited to specific metals. But metals having a specific gravity not less than five nor more than 25 at 20xc2x0 C. can be preferably used. If the specific gravity of the mass-adding part is less than 5, its volume is too large to allow it to have a sufficient mass-adding action. If the specific gravity thereof is more than 25, metal which can be used is rare and expensive or difficult to obtain. Thus the following metals can be used: iron (specific gravity: 7.86), copper (8.92), lead (11.3), nickel (8.85), zinc (7.14), gold (19.3), platinum (21.4), osmium (22.6), iridium (22.4), tantalum (16.7), silver (10.5), chromium (7.19), brass (8.5), and tungsten (19.3). Zinc is harmful. Gold, silver, and the like are expensive. Thus tungsten, copper, nickel, and alloys thereof are preferable. It is preferable to surface-treat the metal powder having a high specific gravity with a coupling agent (for example, silane coupling coating) to allow it to have high degree of adhesion to the macromolecular material.
It is preferable that the diameter of the particle of the metal powder having a high specific gravity is not less than 1 xcexcm nor more than 250 xcexcm. If the particle diameter is less than 1 xcexcm, the metal powder is liable to fly or flocculate. Thus the metal powder is difficult to disperse in the highmolecular compound when they are mixed. If the particle diameter is more than 250 xcexcm, i.e., if the metal powder is large, it is difficult to make the mass-adding part thin.
As the highmolecular compound for the viscoelastic part and the mass-adding part, thermoplastic resin and thermosetting resin are used. The thermoplastic resin includes polyamide resin, polyester resin, urethane resin, polycarbonate resin, ABS resin, polyvinyl chloride resin, polyacetate resin, polyethylene resin, polyvinyl acetate resin, and polyimide resin. The thermosetting resin includes epoxy resin, unsaturated polyester resin, phenol resin, melamine resin, urea resin, diallyl phthalate resin, polyurethane resin, and polyimide resin. The thermoplastic resin is more favorable than the thermosetting resin in consideration of moldability and because it can be recycled.
The thermoplastic elastomer is softer than thermoplastic resin and has higher rubber elasticity, and a lower degree of plastic deformation. Further the thermoplastic elastomer can be recycled. Furthermore it is easy to tune the frequency of the dynamic damper with that of the racket frame. Therefore the thermoplastic elastomer is particularly preferable as the highmolecular compound for the mass-adding part. That is, it is easy to obtain the mass-adding part having a proper complex elastic modulus from a mixture in which the metal powder, having a high specific gravity, serving as the main component is dispersed in the thermoplastic elastomer also serving as the main component.
Although the thermoplastic elastomer is not limited to specific ones, styrene elastomer, olefin elastomer, urethane elastomer, and ester elastomer can be used. The following thermoplastic elastomers are commercially available: Septon compound produced by Kuraray Plastic Corp., Highbla and Septon produced by Kuraray Corp., Elastage produced by Toso Corp., Neat polymer produced by Kanekafuchi Kagaku Kogyo Corp, Nuberan produced by Teijin, Elastomer AR produced by Aron Kasei Corp., Clayton D and Clayton G produced by Shell Japan, Pelprene produced by Toyo Boseki, Toughtech produced by Asahi Kasei Kogyo, Sumiflex, Moldex, Spidex, Sumicon RM produced by Sumitomo Bakelite Corp., Surmoran and Labaron produced by Mitsubishi Kasei Corp., Sumitomo TPE, Sumitomo TPE-SB produced by Sumitomo Kagaku Corp, Epofriend produced by Daicel Corp., Quintack produced by Nippon Zeon, Santoprene and Tolefusin produced by AESxe2x80xa2Japan Corp., and Cirlink produced by DSM Corp.
In the case where the highmolecular compound is used to compose the mass-adding part, it is preferable that the highmolecular compound for the viscoelastic part of the dynamic damper of the present invention is identical or similar to the highmolecular compound for the mass-adding part. Foam material may be used as the highmolecular compound for the viscoelastic part. The highmolecular compound identical or similar to the highmolecular compound for the mass-adding part has a compound melting point to that of the highmolecular compound for the mass-adding part. Thus it is possible to produce the dynamic damper by heating the materials for the viscoelastic part and the mass-adding part in a die to fuse them and integrate them with each other.
The following rubbers are used as the highmolecular compound for the viscoelastic part and the mass-adding part: natural rubber (NR), poly isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR), carboxylated butyl rubber, Isobutylene-Isoprene butyl rubber (IIR), halogenated Isobutylene-Isoprene butyl rubber (X-IIR), ethylene-propylene rubber (EPM), ethylene-propylene-diene rubber (EPDM), ethylene-vinyl acetate rubber (EVA), acrylic rubber (ACM, ANM), ethylene-acrylic rubber, chlorosulfonated polyethylene (CSM), chlorinated polyethylene (CM), epichlorohidrin rubber (CO), urethane rubber, silicone rubber, and fluorinated rubber. If the rubber material for the viscoelastic part and that for the mass-adding part belong to the same family, it is easy to join them to each other by vulcanization, which is suitable for integrating them with each other.
Ethylene.propylene-diene rubber (EPDM) and silicone rubber are preferable because of the weather resistance thereof. Butyl rubber (IIR) is also preferable because of the superior vibration absorption property thereof.
The viscoelastic part and the mass-adding part may be set in a die, with the viscoelastic part and the mass-adding part laminated on each other to form them into a desired configuration. Alternatively the viscoelastic part and the mass-adding part may be formed into a flat sheet and the sheet is formed into a desired configuration by punching the sheet with a punching blade.
According to another aspect of the present invention, a tennis racket having a dynamic damper is installed on at least one portion of a gut-stretched part surrounding a face of a racket frame or/and at least one portion of a throat part of the tennis racket. The dynamic damper has a viscoelastic part and a mass-adding part integrally laminated on each other and is installed on at least one surface of the racket frame in a thickness direction thereof and both surfaces of the racket frame in a widthwise direction thereof. The dynamic damper installed on the racket frame allows for an out-of-plane 2nd damping factor of the racket frame and an in-plane 3rd damping factor thereof to be not less than 1%.
The out-of-plane 2nd damping factor and the in-plane 3rd damping correspond to the damping factor of an out-of-plane 2nd frequency and an in-plane 3rd frequency at the time when the tennis racket deforms in an out-of-plane 2nd mode and an in-plane 3rd mode, respectively. In the case where the value of the vibration damping factor is high (higher than 1%), the dynamic damper favorably damps vibrations generated when the tennis racket hits a ball. Thus a player does not feel uncomfortable vibrations. The dynamic damper has the one part resonating with the vibration of the racket frame in the out-of-plane direction and the other part resonating with the vibration thereof in the in-plane direction. Thus the dynamic damper installed on the tennis racket effectively reduces the vibration in the out-of-plane direction and the in-plane direction and relieves and reduces shock and vibrations sufficiently.
The dynamic damper has the viscoelastic part and the mass-adding part laminated on the viscoelastic part. The dynamic damper is installed on the racket frame via the viscoelastic part. Therefore, the viscoelastic part of the dynamic damper vibrates greatly for the vibration of the tennis racket, thus vibrating the mass-adding part. The dynamic damper vibrates earlier than the racket frame, thus consuming vibration energy of the racket frame and damping the vibration of the racket frame rapidly. Consequently, the dynamic damper can greatly reduce the degree of shock and vibrations to be applied to a player""s hand.
In the dynamic damper, supposing that the top position of the face of the racket frame is 12 o""clock by regarding the face surrounded with the gut-stretched part as the surface of a clock, it is preferable to install the dynamic damper on at least one portion of an angular range of xc2x115xc2x0 with respect to a three o""clock position and on at least one portion of an angular range of xc2x115xc2x0 with respect to a nine o""clock position. The dynamic damper-installed tennis racket does not affect its operability and can reduce the vibration in the out-of-plane direction and the in-plane direction efficiently.
As described above, the vertical frame of the dynamic damper of the present invention mainly resonates with the vibration in the out-of-plane direction, while the horizontal frame thereof resonates with the vibration in the in-plane direction. Thus the dynamic damper can effectively reduce the vibration in both directions and relieves and reduces shock and vibrations sufficiently. The dynamic damper-installed tennis racket has the out-of-plane the 2nd damping factor and the in-plane the 3rd damping factor (hereinafter may be referred to as merely damping factor) at not less than 1% and is superior in its vibration-damping performance. Further because the weight of the entire dynamic damper is suitable for the tennis racket, a player can swing it favorably.
The three o""clock position and the nine o""clock position are maximum amplitude positions of the in-plane vibration and that of the out-of-plane the 2nd vibration. Thus the installation of the dynamic damper at the three o""clock position and the nine o""clock position is optimum for suppressing vibrations in the in-plane and out-of-plane directions and rotation of the grip.
Because a mass is applied to a portion of the gut-stretched part having a large width, the moment of inertia on the grip becomes large. Thus when we hit a ball at the off-center, the dynamic damper prevents the rotation of the racket and reduces the degree of burden to be applied to the player""s elbow owing to the installation of the dynamic damper at the three o""clock position and the nine o""clock position.
As described above, it is preferable that the dynamic damper is installed on the gut-stretched part of the racket frame, with the longitudinal direction of the vertical frame set along the longitudinal direction (direction horizontal to the gut plain of the racket frame) of the racket frame.
In the dynamic damper, supposing that the top position of the face of the racket frame is 12 o""clock by regarding the face surrounded with the gut-stretched part as the surface of a clock, it is preferable to install the dynamic damper on at least one portion of an angular range of xc2x115xc2x0 with respect to a four o""clock position and on at least one portion of an angular range of xc2x115xc2x0 with respect to an eight o""clock position. The dynamic damper-installed tennis racket does not affect its operability and can reduce the vibration of the racket frame in the out-of-plane direction and the in-plane direction efficiently. More specifically, because the balance of the tennis racket can be placed at the side of the player""s hand, the player can swing the tennis racket easily.
In the dynamic damper, supposing that the top position of the face of the racket frame is 12 o""clock by regarding the face surrounded with the gut-stretched part as the surface of a clock, it is preferable to install the dynamic damper on at least one portion of an angular range of xc2x115xc2x0 with respect to a five o""clock position and on at least one portion of an angular range of xc2x115xc2x0 with respect to a seven o""clock position. The dynamic damper-installed tennis racket does not hurt its operability and can reduce the vibration of the racket frame in the out-of-plane direction and the in-plane direction efficiently. More specifically, because the balance of the tennis racket can be placed at the side of the player""s hand, the player can swing the tennis racket easily.
From the viewpoint of balance, it is preferable that the dynamic damper of the present invention is installed on the racket frame at left and right positions symmetrical with respect to the center in the widthwise direction of the racket frame. But the dynamic damper-installing position is not limited to a symmetrical position. A plurality of the dynamic dampers maybe mounted on the racket frame.
It is preferable to make a concavity of the racket on the dynamic damper-installing position.